BW9910 Datasheet, PDF (6/14 Pages) Bruckewell Technology LTD – High Brightness LED Driver

BW9910/A High Brightness LED Driver
Application Information
AC-DC Off-Line Application
The BW9910/BW9910A is a low cost off-line buck or
boost converter control IC specifically designed for driving
multi-LED stings or arrays. It can be operated from either
universal AC line or any DC voltage between 15V and
500V. Optionally, a passive power factor correction circuit
can be used in order to pass the AC harmonic limits set
by EN61000-3-2 class C for lighting equipment having
input power less than 25W. The BW9910/BW9910A can
drive up to hundreds of HB LEDs or multiple strings of HB
LEDs. The LED arrays can be configured as a series or
series/parallel connection. The BW9910/BW9910A
regulates constant current that ensures controlled
brightness and spectrum of the LEDs, and extends their
lifetime, and also allows PWM control of brightness via an
enable (PWM_D) pin.
The BW9910/BW9910A can also control brightness of
LEDs by programming continuous output current of the
LED driver (so-called linear dimming) when a control
voltage is applied to the LD pin.
The BW9910/BW9910A is offered in standard 8-pin SOIC
and SOIC-EP packages.
The BW9910/BW9910A has a built-in high-voltage linear
regulator that powers all internal circuits and can also
serve as a bias supply for low voltage and low power
external circuitry.
LED Driver Operation
The BW9910/BW9910A can control all basic types of
converters, isolated or non-isolated, operating in
continuous or discontinuous conduction mode. When the
gate signal turns on the external power MOSFET, the
LED driver stores the input energy in an inductor or in the
primary inductance of a transformer and, depending on
the converter type, may partially deliver the energy
directly to LEDs. The energy stored in the magnetic
component is further delivered to the output during the
off-cycle of the power MOSFET producing current
through the string of LEDs (Fly-back mode of operation).
When the voltage at the V_DD pin exceeds the V_UVLO
threshold voltage, the gate drive is enabled. The output
current is controlled by means of limiting peak current in
the external power MOSFET. A current sensing resistor is
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connected in series with the source terminal of the
MOSFET. The voltage from the sensing resistor is
applied to the CS pin of the BW9910/BW9910A. When
the voltage at CS pin exceeds a peak current sensing
threshold voltage, the gate drive signal terminates, and
the power MOSFET turns off. The threshold is internally
set to 250mV, or it can be programmed externally by
applying voltage to the LD pin. When the soft-start
function is required, a capacitor can be connected to the
LD pin to allow this voltage to ramp at a desired rate,
therefore, assuring that output current of the LED ramps
gradually. Additionally, a simple passive power factor
correction circuit, consisting of 3 diodes and 2
capacitors, can be added as shown in the typical
application circuit diagram of Figure 6.
Supply Current
A current of 1.0mA is needed to start the
BW9910/BW9910A. As shown in the block diagram on
page 5, this current is internally generated in the
BW9910/BW9910A without using bulky startup resistors
typically required in the off-line applications. Moreover,
in many applications the BW9910/BW9910A can be
continuously powered using its internal linear regulator
that provides a regulated voltage of 7.5V/10V for all
internal circuits.
Setting Lighting Output
When the buck converter topology of Figure 5 is
selected, the peak CS voltage is a good representation
of the average current in the LED. However, there is a
certain error associated with this current sensing method
that needs to be accounted for. This error is introduced
by the difference between the peak and the average
current in the inductor. For example, if the peak-to-peak
ripple current in the inductor is 150mA, to get a 500mA
LED current, the sensing resistor should be as follows :
0.43Ω
Dimming
Dimming can be accomplished in two ways, separately
or combined, depending on the application. Light output
of the LED can be controlled either by linear change of
its current, or by switching the current on and off while
maintaining it constant. The second dimming method
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